A thin film transistor (TFT) has been widely used as a switching device for a display such as a liquid crystal display device or the like. A representative TFT has a configuration in which a gate electrode, an insulator layer and a semiconductor layer are stacked in this sequence on a substrate, and has, on the semiconductor layer, a source electrode and a drain electrode being formed with a predetermined interval therebetween. The semiconductor layer constitutes a channel part, and an on-off operation is conducted by controlling electric current flowing between the source electrode and the drain electrode by a voltage applied to the gate electrode.
Conventionally, the semiconductor layer of such a TFT was fabricated by using amorphous or polycrystalline silicon. However, a CVD apparatus used for fabrication of a TFT using silicon is very expensive, and an increase in size of a display or the like using a TFT had a problem that the production cost increased significantly. Further, there was a problem that, since forming amorphous or polycrystalline silicon into a film is conducted at significantly high temperatures, the type of a material which is usable as a substrate is limited, and hence, a lightweight resin substrate or the like could not be used.
In order to solve the problem, a TFT using an organic substance (hereinafter often abbreviated as an organic TFT) instead of amorphous or polycrystalline silicon has been proposed. As the film-forming method which is used when a TFT is fabricated by using an organic substance, a vacuum vapor deposition method, a coating method or the like are known. According to these methods, it is possible to realize an increase in size of a device while suppressing an increase in the production cost, and is also possible to allow the process temperature which is required during film formation to be relatively low. Accordingly, such an organic TFT has advantages that only small restrictions are imposed on the type of materials used for a substrate. Therefore, its practical use has been promising and research reports have been actively found.
A practical TFT is required to have a high carrier mobility, a large current on/off ratio and excellent storage stability. Meanwhile, the on-off ratio as referred to herein means a value which is obtained by dividing a current which flows between source and drain electrodes when a gate voltage is applied (ON) by a current which flows between source and drain electrodes when a gate voltage is not applied (OFF). The on current is normally means a current value at the time when the current flowing between source and drain electrodes is saturated (saturation current) after increasing the gate voltage.
As the organic substance used in an organic semiconductor layer of a TFT, as for a p-type organic TFT, polymers such as conjugated polymers and thiophene-based oligomers (Patent Documents 1 to 5 or the like), fused aromatic hydrocarbons such as pentacene (Patent Documents 6 and 7 or the like) and the like are used singly or in a mixture with other compounds.
However, these organic TFTs have a problem that the contact resistance between the source electrode and the organic semiconductor and/or between the drain electrode and the organic semiconductor is high, and hence, they require a high driving voltage. Further, there is a disadvantage that the too large contact resistance results in a low field effect mobility and a low on/off ratio.
In general, in an organic TFT, carriers are injected from a source electrode by the application of gate voltage, and a channel is formed in an organic semiconductor. By applying a voltage (drain voltage) between the source electrode and the drain electrode, current (drain current) is flown between the source electrode and the drain electrode. By the gate voltage, the amount of carriers injected changes, whereby the drain current can be controlled. Quantitatively, the drain current can be expressed by the following formulas (1) and (2). The formula (1) is satisfied in a linear region in which the drain voltage is small, and the formula (2) is satisfied in a saturation region in which the drain voltage is large.
                              I          D                =                                            W              ⁢                                                          ⁢              μ              ⁢                                                          ⁢              C                        L                    ⁢                                    V              D                        ⁡                          [                                                (                                                            V                      G                                        -                                          V                      th                                                        )                                -                                                      1                    2                                    ⁢                                      V                    D                                                              ]                                                          (        1        )                                          I          D                =                                            W              ⁢                                                          ⁢              μ              ⁢                                                          ⁢              C                        L                    ⁢                                                    (                                                      V                    G                                    -                                      V                    th                                                  )                            2                        2                                              (        2        )            wherein ID: drain current, VD: drain voltage, VG: gate voltage, Vth: threshold voltage, μ: field effect mobility, C: capacity of an insulator per unit area, L: channel length, and W: channel width.
In the above formula, if the organic semiconductor is in the ideal state, that is, in the state where the source electrode and the drain electrode are in Ohmic contact and no carrier injection barriers are present, the field effect mobility μ will be close to the value which is inherent to a material. However, in general, contact resistance is generated between the organic semiconductor and the metal electrode. In the region where the drain voltage is small, the relationship between the current and the voltage is deviated from the relationship shown by the formula (1), and hence, the switching characteristics in this region are not good. Further, the voltage drops in the interface between the metal and the organic semiconductor, and an effective voltage applied to the organic semiconductor is decreased in an amount corresponding to the dropped voltage. As a result, the field effect mobility μ in the formulas (1) and (2) is calculated in a smaller value, causing problems such as lowering of the response speed or the on/off ratio, an increase in driving voltage or the like. Injection of carriers as referred to herein means, in the case of a p-type organic TFT, injection of holes from the electrode to the HOMO level, and in the case of an n-type organic TFT, injection of electrons from the electrode to the LUMO level. If contact resistance is generated from the injection barrier, in order to decrease the resistance as much as possible, in a p-type organic TFT, a metal having a large work function is used as the source electrode and the drain electrode in an attempt to decrease the injection barrier of holes. In many cases, Au (work function: 5.1 eV, page 493, Kagaku Bin ran II, (Handbook of Chemistry) Revised edition 3, published by Maruzen Publishing Co., Ltd. on 1983) is used. However, the HOMO level of many organic semiconductors which exhibit excellent performance as the organic TFT is larger than that of Au, and even though Au is used, contact resistance is generated due to the presence of the injection barrier, and as a result, as mentioned above, problems such as an increase in driving voltage, lowering of mobility, decrease in on/off ratio or the like arise.
In order to solve this problem, Patent Document 8 discloses that the source electrode and the drain electrode respectively contain a carrier relay film and a carrier conductive film, and a metal which constitutes the carrier relay film which is in contact with the organic semiconductor has a work function which is close to the ionizing potential of the organic semiconductor. Further, Patent Document 9 discloses an organic thin film transistor in which a charge injecting layer formed of an inorganic substance is interposed between the source electrode and the drain electrode and the organic semiconductor film. However, by using these disclosed materials, although a slight decrease in driving voltage is possible, a transistor has performance which is still insufficient for practical use.
On the other hand, an attempt has been made to produce an electrode easily by using a conductive polymer. Non-Patent Document 2 discloses an organic TFT in which the source electrode and the drain electrode are formed by the ink jet method by using PEDOT:PSS. This document also states that contact resistance is generated between these electrodes and the polymer F8T2 used as the organic semiconductor. PEDOT:PSS still has a problem for use as the source electrode and the drain electrode. Further, as a conductive polymer which has advantages such as being stable in the air, in Patent Document 10, as an electrode material which injects holes to an active layer, the following is disclosed as an example of using polyaniline (PANI) which is a compound similar to that used in the invention. That is, Patent Document 10 discloses an organic EL device in which a 200 nm-thick coating film of polyaniline having a specific conductivity of 50 S/cm and an area resistance of 1 kΩ/□ is formed on a substrate. However, this document states only the conductivity, and does not state which materials are excellent as the electrode. In addition, Patent Document 11 discloses an organic TFT which can select polyaniline as the gate electrode or the source electrode. As example of such electrode, polyaniline derivatives are given. However, no specific statement is made as to which material of these polyaniline derivatives can improve the device properties. As the patent in which polyaniline is used as an electrode, Patent Document 12 can be given. In Example 3 of this patent, conductivities of pellets obtained by compressing polyaniline which is protonated by using various protonic acids such as dioctyl sulfosuccinate are shown in Table 1. Example 54 and after disclose an organic light-emitting diode (organic EL) using polyaniline in which CSA ((±)-10-camphorsulfonic acid) is used as a dopant is used as a hole-injecting electrode (anode). In this Example 54, there is a statement that current flowing a device using a conductive polyaniline transparent film as the hole-injecting contact is almost the same as that flowing a device fabricated using ITO as the hole-injecting contact. No improvement for hole-injecting properties relating to a lowering in driving voltage has been made.